This application is directed to vacuum microelectronic devices and methodology for producing such devices, having first and second substrates capable of withstanding heat and pressure; at least one of the substrates having at least one cavity with electrodes deposited in the cavity; the first and second substrates wafer bonded together such that the substrates are joined together at all points of contact.
Lamps
Gas discharge lamps (mercury vapor, sodium vapor, metal halide) are an important segment of the lighting industry. It is well known that the luminous efficacy of gas discharge lamps increases substantially at high pressures (1-200 atmospheres). However, the containment of such high pressures in a transparent vessel has presented significant problems. Gas pressure is restricted in many instances because of the difficulty of finding materials that are sufficiently lightweight, while at the same time capable of withstanding high heat and pressures. Furthermore, such materials, to be practicable, must be capable of relatively inexpensive mass production. The usual construction of gas discharge lamps is to suspend a translucent pressure and heat resistant discharge vessel by means of a metal framework within an outer glass bulb.
The present invention provides an entirely new paradigm for the construction of high pressure gas discharge lamps and displays. Rather than a discharge vessel mechanically suspended within an outer bulb, the present invention is directed towards methods of fabricating high pressure "microlamps" utilizing micromachining techniques which are similar to integrated circuit fabrication techniques such as the etching of and bonding of planar substrates. The present invention is directed to an improved gas discharge lamp that can withstand very high pressures and the method of making such a lamp by means of integrated circuit manufacturing techniques. The lamp is manufactured from two planar sheets of temperature and pressure resistant transparent material. A cavity is etched in one or both of the sheets and electrodes are therefore deposited or placed in the cavity. The cavity is charged with a filler appropriate to the type of lamp being manufactured such as mercury, sodium or metal halides. The two sheets are then bonded together so as to seal the cavity within the sheets. Contact may then be made with the electrodes to activate the lamp. Electrodeless lamps activated by microwave energy may also be manufactured by this technique. Miniature gas discharge lasers may also be produced by this technique.
The term "bonding" as used throughout this application refers to "wafer bonding" techniques used in the manufacture of integrated circuits and sensors. Such techniques generally comprise anodic or fusion bonding which results in a chemical bond at the interface which is as strong as the bulk material. This bond permits the fabrication of cavities which can withstand extremely high pressures (greater than 200 atmospheres). In this technique, a bond is present at all points of contact between the substrate surfaces, so that each cavity is individually sealed.
There are two types of wafer bonding processes. Anodic wafer bonding or fusion wafer bonding. Fusion wafer bonding: In this process two flat wafers (e.g. quartz) are prepared with hydrophilic surfaces and brought into contact. The Van de Waal's forces pull the two wafers together and result in a bond at the interface. The two wafers are then annealed at high temperature (e.g. 1000 C.), resulting in a chemical bond at the interface, which has the strength of the bulk material. Even though the temperature is elevated, bonding takes place at a temperature below the melting point of the material (quartz: approximately 1400.degree. C.). This means that the substrate will not deform during the bonding process. Anodic wafer bonding: In this process, two flat wafers are brought into contact as in the fusion wafer bonding process. However, the annealing is carried out at lower temperatures and with an electric field applied across the wafers. This process is useful for materials that have mobile ions and cannot be annealed at high temperatures (such as glass). The electric field results in the collection of positive and negative charges at the interface, which lead to high electric fields, which pull the wafers together. This process is more forgiving of the degree of wafer flatness, but is more difficult to implement and does not work with materials that are free of mobile ions.
The present development has implications for fluorescent lamps as well. Fluorescent lamps fabricated by using wafer bonding will not only be much sturdier, but the internal pressures can exceed 1 atmosphere. This is because flat fluorescent lamps made by the present technology will be bonded everywhere, except where the discharge space is located. As noted above, this bond will have the strength of the bulk material. This allows the lamp to be optimized in a sub-normal "glow" discharge mode, where the optimum pressure for the discharge can exceed 1 atmosphere for short electrode spacing or narrow walls. Glass frit sealing (at the edges) in the prior art is a mechanical process, which cannot easily be used to make small individually sealed cavities (e.g. 100 micron diameter) and which is not a batch production process. On the other hand, the wafer bonding process can be used with IC techniques to make extremely small individually sealed cavities (&lt;100 micron diameter) and is a batch process which is fully compatible with IC techniques. The present development also permits the phosphors that fluoresce to be located either inside the discharge cavity (as in current fluorescent tubes) or outside of the cavity.
Displays
Emissive, large area displays are expected to have applications in the large flat panel television market, particularly for high definition television. Plasma displays are promising candidates for this application. Plasma displays are constructed by patterning orthogonal electrodes on glass substrates, placing spacers between the glass substrates, sealing the substrates at the periphery and filling the space in between the substrates with the working gas. In another version of a plasma panel, a glass sheet with holes cut through it is placed between electrode substrates. As a result, individual discharges are confined to the holes in the middle sheet. However, the seal is still made at the periphery and the gas is filled from a small opening at the periphery, which is thereafter sealed. In both types of structures, however, the individual pixels are not sealed from each other. Such conventional plasma displays or back-lights are shown in Patent Publications WO 90/09676, WO 87/04562, EP 0,302,748 and EP 0,467,542 A2.
Since the conventional displays are large, the gas fill pressure cannot be much more than one atmosphere, as the forces on the glass substrate are too great and will either force the substrates apart, or shatter them. For example, in a panel measuring 1200 sq in (30.times.40 in), only 0.1 psi of pressure over 1 atmosphere will result in an outward force of 120 lbs on each substrate. Therefore, the gas fill in these substrates must be restricted to a maximum pressure of 1 atm. At the same time, one of the key problems plaguing the plasma panels is low brightness for TV applications because of the lower pressure at which they must be operated.
This application is directed to a display technology based on micromachining, that will yield bright, efficient and rugged displays for TV applications. In this construction, the individual pixels are completely isolated from each other. The individual pixels are formed by etching and bonding of the transparent substrates (glass, fused quartz, sapphire, PCA or others), so that the pixels are comprised of sealed cavities containing electrodes and the ambient gas or dosing material. Since each pixel is a sealed cavity, which is formed by bonding the etched substrates, the bonded interface is as strong as the bulk of the substrate and the pressure within such a pixel can be substantially more than one atmosphere.
The improved construction can readily provide higher pressure discharges, which can emit substantially more light. This is particularly advantageous for HDTV applications, where pixel sizes need to be small (&lt;300 .mu.m), even for large area displays. However, it is difficult to use mechanical processes economically to make small but deep apertures. Furthermore dry chemical etching processes are too slow and also too expensive. Therefore, the pixel cavities must be made by wet chemical etching, which is isotropic and results in etched cavities whose lateral dimensions are roughly twice the depth. Therefore, the spacing between electrodes must be kept small. However, in order to optimize the UV radiation from xenon, it has been demonstrated that the ratio of electric field to pressure (E/p) is 7-8 V/cm.torr. Therefore, for driving voltages of 60 V and electrode spacings less than 100 .mu.m, the optimum pressure is more than one atmosphere which is not obtainable with prior manufacturing processes.
Although, the discussion so far has assumed that the discharges are in the "glow" phase, another advantage of this technology is that the pixels can be operated as high pressure arc lamps. It is well known that these high pressure arc lamps have significant output in the visible wavelength range of the electromagnetic spectrum. Therefore, the pixels can be used with color filters to form a display, thus avoiding the use of phosphors. When the discharge is operated in the "glow" phase, the electrodes do not get very hot, however, when the discharge is operated in the "arc" phase, the electrodes get hot, thus the choice of the metal for the electrodes is a function of the desired operating range. Additionally, in the arc phase, reignition is a problem, but it can be avoided by use of an auxiliary discharge, which is hidden from view, but connected to the display discharge space as was done in prior panels.
U.S. Pat. No. 4,990,826 is directed to a display device. In this patent, channels are formed by etching through one plate and placing it in contact with two plates on which electrodes have been formed. The three plates are then sealed together by heating. Because of the electrodes, the seal can only be formed by softening the glass so it can flow around the metal electrode and form a seal. Such heating may unacceptably deform the entire structure. This patent, a vacuum port is also sealed into the device with glass frit. The discharge space is evacuated and filled through this port and then sealed by melting the pumping port tube.
The discharge space in the present invention is formed by wafer bonding two or more plates together. In the present electroded lamps, the electrodes are sealed and the surface of the wafer is planarized by using wafer planarizing methods to deposit glass or SiO.sub.2 films. The planarized wafers are then bonded together to seal the discharge space. Since the wafer bonding is carried out in the atmosphere required in the discharge space (e.g. Ar, Ne, Xe, etc.), the atmosphere is sealed into the cavity and a port is not required. Therefore, the present process allows the formation of many individually sealed cavities on one substrate, whereas the process described in U.S. Pat. No. 4,990,826 does not. The present process has the advantages of batch processing whereas the prior art does not. The formation of a seal by wafer bonding also has practical advantages over glass softening, in that the structure will not deform during wafer bonding but will during softening of the glass as in U.S. Pat. No. 4,990,826.
Vacuum Microelectronic Devices
Vacuum Microelectronics is the name given to the emerging technology of microelectronic "vacuum state" devices. Since these devices are based on the motion of electrons in vacuum, they are expected to be much faster than solid state devices in which the electron drift velocity is slower. In addition to higher speed, vacuum microelectronic devices are expected to be significantly more radiation hard. In the old vacuum "tube" technology, the electrons moved in a vacuum, but the devices were much larger and transmit distances much greater, so that technology was quickly replaced by solid state technology, where solid state devices could meet the performance requirements. Therefore, if "vacuum state" devices could be downsized to the small-size of solid state devices, it would be a significant advantage.
The basic vacuum tube device, the classic triode, however, is based on thermionic emission and it is generally believed that the high temperatures and high power dissipation required for satisfactory operation are not acceptable in any microelectronic structure. Therefore, the vacuum microelectronic devices developed to date are based on a "cold" emission process: field assisted emission. These devices are based on emission from sharp points of suitable materials or from reverse biased shallow junction diodes. The equivalent of the cathode is the pointed emitter (cold cathode), which emits electrons, which are collected by an anode and modulated by a grid. These devices are in the early stages of research and have yet to demonstrate significant results. The structures that have been fabricated have to be placed in a vacuum chamber to be tested.
This application is directed to a new technology for vacuum state devices, which not only allows the fabrication of individually sealed vacuum microdevices, in which the vacuum space is closed and sealed so that a vacuum chamber is not required for their operation, but also allows the fabrication of vacuum microdevices based on thermionic emission (hot cathodes), which operate at high temperatures, which is not the case with existing approaches to vacuum microelectronics.
This technology is based on the formation of cavities in one or both of a pair of substrates, followed by alignment and wafer bonding of the substrates to seal the cavities. The basic structure, which is the equivalent of the traditional triode, is fabricated by using a process sequence similar to those used in our previous applications to form light emitting devices. The substrate may be of any material suitable for wafer bonding, such as quartz, sapphire, silicon or glass, depending on the anticipated operating conditions of the device. The wafer bonding process used can either be fusion bonding or anodic bonding, also depending on the substrate. Similar bonding processes are widely used in sensor applications.
In the first step, a first electrode of an electron emissive material, such as W, Mo, other refractory metal or a silicide, is deposited on a first substrate and patterned. An intermediate layer of silicon dioxide is then deposited (by CVD, PECVD, SILOX or other methods) followed by another electrode which is deposited and patterned. The distance between the first and second electrodes can be adjusted simply by varying the thickness of the silicon dioxide. This distance will be, for example, the distance between the cathode and the grid. A second intermediate layer is then deposited, and the wafer is then planarized by reflowing of the layer or by any other planarizing method. A cavity is then etched around the electrodes, by using photolithography and selective etching.
A third electrode, for example, the anode, is deposited on a second substrate. The second substrate is then planarized, aligned and bonded to the first, in a vacuum environment, resulting in a sealed cavity containing a vacuum environment. The fusion or anodic bonding results in a chemical bond at the interface, which has the strength of the bulk material. Contact holes to the electrodes are then opened up by either etching or "drilling" with a laser.
Preferably, the third electrode is deposited and patterned inside a trench, which is etched on the second substrate. The trench is then filled, and planarized, after which a cavity surrounding the electrode is formed by photolithography and etching, prior to bonding.